A nurse is talking to and making facial expressions at a 9-month-old baby girl during a routine office visit. Which of the following represents the most advanced milestone of language development that the nurse should expect to see in this child?
a) The infant says "da-da" when looking at her father
b) The infant coos, babbles, and gurgles
c) The infant squeals with pleasure
d) The infant imitates her father's cough
Answer: The infant says "da-da" when looking at her father